CMOS Inverter | Sheet Resistance | VLSI | Lec-38

TL;DR

This video teaches how to calculate the on resistance for CMOS inverters.

Transcript

hi everyone in this video you are going to learn how to calculate the on Resistance from vdd to VSS when we go for the CMOS inverter in the previous video I have explained you how to calculate the on resistance for the nmos inverter and for n Channel transistor okay on resistance for n Channel transistor we have seen thereby on resistance for the t... Read More

Key Insights

• ✊ Understanding on resistance is vital for power management in digital circuits, especially in CMOS technology.
• 🎨 The technology node (e.g., 5 micrometers) establishes baseline values for resistance calculations, impacting design choices.
• ❓ The relationship between channel length and its effect on on resistance must be considered when optimizing inverter performance.
• 🦻 The table provided enhances clarity by showing resistance values for various transistor types and technologies, aiding quick reference during calculations.
• ❓ The calculation method remains consistent across NMOS and PMOS, underscoring the importance of understanding each transistor's parameters.
• 🪜 Knowing how to add the on resistance values from both types of transistors is essential for determining the total on resistance of CMOS inverters.
• 🎨 Effective inverter design requires not just resistance values but also an appreciation of layout and fabrication technologies' impacts on performance.

Questions & Answers

Q: What is the significance of calculating on resistance in CMOS inverters?

On resistance in CMOS inverters is crucial for understanding how much resistance the inverter presents between the power supply (Vdd) and ground (VSS). This directly influences the inverter's speed, power consumption, and overall performance in digital circuits. Calculating it accurately allows engineers to design more efficient circuits.

Q: How does the channel length of transistors affect the on resistance?

The channel length of transistors, denoted by different technology nodes (like 5 micrometers), directly affects on resistance values. Shorter channel lengths generally lead to lower on resistance, enhancing the speed of the inverter. However, they might also introduce other challenges such as increased leakage current.

Q: Can you explain the formula used for calculating on resistance?

The on resistance is calculated using the formula R = Z * RS, where Z is the length-to-width ratio (L/W) of the transistor, and RS is the sheet resistance per square. This formula allows for adjustments based on the dimensions of the NMOS or PMOS transistors, influencing the total resistance experienced in the CMOS inverter.

Q: What are the typical values for sheet resistance in 5 micrometer technology for PMOS and NMOS transistors?

In 5 micrometer technology, the sheet resistance for PMOS transistors is typically 2.5 x 10^4 ohms per square, translating to 25 kilo ohms, while for NMOS transistors, it is often 1.0 x 10^4 ohms per square, or 10 kilo ohms. These values are crucial for calculating the on resistance of individual transistors.

Summary & Key Takeaways

• The video provides a step-by-step guide for calculating the on resistance from Vdd to VSS in CMOS inverters and focuses on NMOS and PMOS transistors.

• It discusses the significance of technology nodes like 5 micrometers and explains how dimensions affect the resistance calculations for both types of transistors.

• Detailed examples are given for calculating the on resistance using standard formulas and resistance values per square for different technologies, leading to a total on resistance value.